A lithium polymer battery or a lithium gel battery is known. See, for example, U.S. Patent Publication No. 2002/0168564A1 which is incorporated herein by reference. These lithium polymer batteries are different from lithium-ion secondary batteries. Two significant differences are: the use of a gel electrolyte versus the use of a liquid electrolyte, and the flexible packaging for containing the active battery elements versus the rigid can (button or cylindrical or prismatic) for containing the elements.
Originally, the hope of polymer battery developers was to use a gel electrolyte as the separator. That is the gel electrolyte would separate the anode and the cathode. But, that scheme has been left behind for at least two reasons. First, the gel electrolyte did not provide any shutdown capability, a safety feature to reduce the occurrence of catastrophic short-circuiting. Second, the soft gel electrolyte was difficult to handle in the battery manufacturing process. Batteries are typically made in a continuous operation from tapes of the elements and a tape of gel did not have sufficient mechanical strength to withstand the rigors of the manufacturing process.
Now, the gel electrolyte separator is replaced with a gel polymer coated microporous membrane that is subsequently impregnated with a liquid electrolyte. The inclusion of the membrane overcomes the aforementioned problems with the gel electrolyte separator. The membranes can provide the shutdown capability and have the mechanical strength to withstand the rigors of the battery manufacturing process. Exemplary coated separators are described in U.S. Pat. Nos. 5,639,573; 5,681,357; 5,688,293; 5,750,284; 5,837,015; 5,853,916; 6,322,923; 6,328,770; 6,337,101; U.S. Patent Publication No. 2002/0168564; WO99/54953; WO01/39296; and WO01/39297, each is incorporated herein by reference.
In U.S. Pat. No. 6,322,923 and U.S. Patent Publication 2002/0168564, the separator comprises a microporous membrane having a coating. The coating is made from a mixture of a gel forming polymer, a plasticizer, and a solvent. The solvent dissolves the gel forming polymer and the plasticizer so that the mixture may be easily and evenly applied to the membrane. Also, the solvent is relatively volatile, compared to the other components, so that it may be easily removed. The remaining coated separator (i.e., coating comprising gel forming polymer and plasticizer) is not porous and is not ready to be impregnated with electrolyte until it is made porous. The plasticizer is the pore-forming agent. The plasticizer, for example an ester-base phthalate or an organic carbonate, must be extracted to form the pores. This extraction step adds to the cost of the separator.
Accordingly, there is a need for a less expensive coated separator for use in a lithium polymer battery.